1. Field of the Invention
The present invention relates to a semiconductor device and a method of controlling power supply voltage which drive an internal circuit based on a plurality of power supply voltage. For example, separately from a power supply voltage for a logic circuit, the present invention aims at a semiconductor memory device and so on in which a power supply circuit for a step-up circuit and a step-down circuit is required.
2. Related Background Art
A power supply potential detecting circuit for detecting that an external power supply falls to a certain level and generating a reset signal is conventionally known. The reset signal is an important signal which surely initializes an internal circuit and stops operation of an internal power supply generating circuit, when the external power supply stops unexpectedly, irregularly and instantaneously, or is turned on/off unstably. The power supply potential detecting circuit for generating the reset signal is, for example, composed in manner shown in FIG. 21.
FIGS. 22A-22B are potential wave form diagrams of the circuit of FIG. 21. A waveform w13 of FIG. 22A shows a waveform of an external power supply VDD, a waveform w14 shows a waveform of VDD-Vtp of a PMOS transistor Q21, and a waveform w15 shows an output potential of the circuit of FIG. 21.
In FIG. 21, when the potential of the external power supply VDD falls from the constant potential state at time t1 of FIG. 22A, a gate potential and a drain potential (node 12) of a PMOS transistor Q21 with a threshold voltage Vtp gradually falls from VDD-Vtp as shown in FIG. 22B. The potential of the node 12 falls, and at time point t2 when the potential of node 12 becomes lower than a threshold voltage of an NMOS transistor in an inverter IV21, the output potential 14 is turned off as shown in FIG. 22B.
The power supply potential detecting circuit as shown in FIG. 21 is provided by each power supply when there are a plurality of external power supplies provided in the semiconductor device.
However, when using the semiconductor device in which two different external power supplies VDD and VCC are provided, it is likelihood that one or both of external power supplies may stop unexpectedly, irregularly and instantaneously, or break instantaneously. For example, when one external power supply stop or break instantaneously, the external power supply with higher potential among the power supplies VDD and VCC rapidly falls, or the potential of the external power supply with higher potential falls prior to the external power supply with lower potential. Therefore, the potential relationship may be in reverse. When such a reverse of the potential relationship occurs, a junction leak occurs or the internal circuit may malfunction.